HYDRO AFRICA 2003

Optimised solutions for existing and new spillways

F. LEMPERIERE - X. DE SAVIGNAC

HYDROCOOP-FRANCE

Abstract : The specific flow of ungated spillways may be multiplied by 4 with a new solution: the P.K. Weir (Piano Keys Weir). This solution is not patented and may be implemented easily at low cost within each country.

This safe solution will reduce the cost of most new dams.

It is also possible to use it, as well as some other low cost solutions, for increasing the spilling capacity or raising the operating level (and power output) of many existing small or large dams.

1. THE FLOW OF UNGATED SPILLWAYS MAY BE MULTIPLIED BY 4 AT LOW COST:THE P.K. WEIR :

Ungated spillways are simpler and safer than the gated spillways but their drawback is the reduced specific flow which, for usual sill shapes is (in m³/s/m) about 2,2 H^1,5, H being in m the spilling nappe depth.

Some dozens of spillways used vertical walls with a trapezoidal labyrinth layout for multiplying by about 2 the specific flow but the relevant structures can not be placed on top of gravity dam cross sections .i.e on top of most spillways.

A totally new labyrinth shaped design has been developed since some years by Hydrocoop and tested in hydraulic laboratories in France, Algeria and India. Most of the walls are inclined, the layout shape is rectangular, the average height of the wall is half of the height of traditional labyrinths ; it is very cost effective to multiply by 1.5 to 5 the specific flow of traditional spillways. The lay out is rather similar to Piano Keys, justifying the name of Piano Keys Weir (P.K. Weir).

An example of such solution is given in fig 1 here annexed. It has been optimised taking care of hydraulic tests, structural design and construction facilities.

Increase of specific flows and depth savings as compared with a traditional Creager weir are given hereunder for the model represented in fig. 1 and for a value of H (height of walls) of 8m.

They may be adjusted to the value H ; the depth saving is proportional to H and the specific flows to H ^1,5 , It is thus easy to choose H for a required saving in depth or a required increase in specifie flow

Increasing (or reducing) by 10% the width of elements and keeping same length and height reduces (or increases) by about 5% the savings.

Savings in nappe depth (7) are close to 3,4m or 0,45 H

Increases in specific flow (4) are about 31 m³/s or 1,4 H ^1,5

For specific flows up to 20 m³/s/m the walls can be made in precast simple elements and require about 2 m³ of reinforced concrete for 1^m nappe depth saving along 1 m of spillway..

For higher specific flows (up to 100 m³/s/m) the necessary quantity may increase up to 5 m³ but the cost of the structure will not be higher than the cost of civil works of a spillway using radial gates ; the reinforced concrete remains less than 0,5 m³ par m³/s of spillway capacity increase.

It is possible to increase further the efficiency of P.K. Weirs by 10 to 20 %. More details and other examples are given in the Issue of Hydropower and Dams published in October 2003.

2. Other low cost solutions:

Other low cost solutions may often improve existing dams:

2a Parapet walls upon embankment crest (fig 2)

If the embankment crest is rather impervious, a parapet wall 1m or 1,5 m high in concrete or gabioes may be very cost effective if the dam is not too long: it has been used for increasing to some extent the safety of many dams.

2b R.C.C. downstream slope lining (fig 3)

For one hundred low earth fill dams in U.S., the downstream slope has been lined with roller compacted concrete in order to allow overtopping. This may be a low cost solution for emergency spillways to be placed where the dam height is between 5 and 10m with a nappe depth close to the freeboard..

2c Flashboards (fig 4)

These fuse elements are used for thousands of small reservoirs in United States. They are usually made by wood boards standing against vertical steel pipes anchored in the spillway sill. The elements may be withdrawn during most of the flood season ; they are overtopped by ordinary floods and bend in sequence for exceptional floods. Using simple steel plates anchored in the sill may be an alternative. The height of flashboards is usually 1 ^m or less.

2d Concrete fuseplugs (fig 5)

Various fuse devices have been used, including fuse gates which tilt for a very precise upstream level when uplift is created under them (Hydroplus).

A simpler solution is made by concrete fuseplugs supporting full uplift and tilting for an upstream level according to the fuseplugs thickness. The cost is low ; the precision is less than with fusegates but this is acceptable for instance for increasing the safety of free flow spillways.

3. NEW DAMS:

In most African countries the exceptional floods may be very high and the advisable level of safety increases the capacity i.e the cost of new spillways which may thus be a large part of the total dam cost.

Well adapted risk analyses and new solutions for spillways may allow substantial savings.

Thirty years ago, most dams were designed for a "design flood" of a yearly probability such as 10^-3 Above the relevant reservoir level a margin of safety of few meters (the free board) was kept under the embankment crest but the true probability of overtopping (and failure) was not analysed precisely.

It is now usual and advised by ICOLD bulletins to consider also a "check flood" of very low yearly probability (such as 10^-5) or a theoretical "Probable Maximum Flood" and to make sure that this check flood (often about twice the design flood) may be spilled without dam failure ; but it is usually accepted that the corresponding reservoir level may be close to the embankment crest as the flood failure requires usually an overtopping of some hours by a nappe depth of 0,20 to 0,50^m. the freeboard is thus very reduced for the check flood.

Two basic solutions have been used for spillways:

- Gated spillways most often preferred for capacities over 1000 m³/s, radial gates being the most usual. The reservoir may be operated at the level of the design flood. The area of gates is adapted to the check flood, i.e. to about three times the flood of yearly probability 10^-2. consequently during a century two thirds of the spillway capacity will probably never be used ; however this solution is not fully safe because it requires careful maintenance and operation and redundancy of operating devices (including power supply). Incidents during heavy rains are not uncommon and total jamming of gates has caused the failure of some large dams: this risk appears the largest risk of gated dams andjamming may be caused by mechanical or electrical problems, accesses, lack of operators, will full damage. For new dams in case of total gates jamming it should be possible to spill over the gates or through an emergency spillway at least the yearly flood i.e. for instance twenty per cent of the check flood. This is obtained if the freeboard above the gates is half of the gates height, the flow over the gate being (1/3)^1,5 i.e. about 20% of the flow with all gates open.

In order to avoid the operating cost of gates various solutions of automatic gates have been used worldwide but their reliability is questionable as two risks may happen: unnecessary opening or total jamming. Consequently they should be used only for small reservoirs or for a reduced part of the total spillage capacity.

- For avoiding the cost or risks of gates, two thirds of the world large dams and particularly most spillways under 1000 m³/s capacity are free flow spillways. Their operation is simple and safe but the drawback of the usual shapes such as the Creager shape is the rather low specific flow : the flow per ml of spillway length (in m³/s/m) is about 2,2 H^1,5, H being the nappe depth in m. A flow of 1000 m³/s under 3^m depth requires thus 90^m of spillway length. Beyond the cost of a long spillway for embankment dam, the maximum nappe depth reduces accordingly the useful reservoir depth ; and reducing for instance by 3 meters a reservoir depth of 30^m reduces in fact by about 30% the live storage or increases by 20% the dam cost. The reduction in hydropower output may also be important. Multiplying by 3 or 4 the specific flow of the spillway if using the P.K. Weir solution will be most often a key improvement. Examples are given hereunder for spillways of 200m³/s, 1000m³/s and 5000 m³/s.

For 200 m³/s, a traditional spillway would be for instance 35^m long and the nappe 2^m deep. With a P.K. Weir the length may be reduced to l0^m avoiding a side spillway to embankment dams. This would require about 60m³ of reinforced concrete in precast elements, less than 20 000$ in most African countries.

For 1000 m³/s, instead of a 90^m spillway 3^m deep it is possible to use a 20^m long spillway of same depth or a 40^m long spillway with a nappe depth of 2^m, using 300m³ of reinforced concrete in precast low cost elements.

For a spillway of 2500m³/s of design flood and 5000 m³/s of check flood, a traditional design would use for instance 4 radial gates 12^m wide and 10^m high with a free board of 5^m used for the check flood. The total length of the spillway, including piers will be 60m. In case of total gates jamming the maximum flow over the gates will be 1000 m³/s, less than the yearly flood. Three alternatives using P.K. Weirs are suggested hereunder.

- A P.K. Weir 80m long and 10m high at the same level as the top of the gates. For the check flood, the level would be the same. For a yearly flood, the nappe depth of the P.K. Weir would be 0 ^m,80, and for the 100 years flood about 1 ^m,50. it would be thus necessary to buy more land than with the gated dam but the overall cost will be much lower and the safety improved. It would require 2000 m³ of reinforced concrete.

- A.P.K. Weir 50m long and 2 flap gates 6m high and 12^m wide. This would add flexibility for managing the reservoir and controlling floods. The gates could be automatic. For the 100 years flood the reservoir level would be 1 m above the normal operating level.

- A.P.K. Weir 40m long and 2 radial gates as in the basic solution. The length of the spillway and operating levels are quite the same as for the basic solution for all floods. This solution is substantially less expensive than the basic one. The total cost of civil works is about the same and half of the cost of gates is avoided. In case of total gates jamming it is possible to spill the design flood (much more than in the basic solution).

For most new dams, at last one solution using P.K. Weirs will be thus less expensive than traditional solutions, when keeping or improving the safety. As there are no patents, these solutions may be easily implemented with resources of each country.

4. INCREASING THE OPERATING LEVEL OF EXISTING FREE FLOW RESERVOIRS.

It will be possible to lower the existing spillway sill and to raise a P.K. weir reducing the maximum nappe depth by half.

Saving 1 m depth will require about 2m³ of reinforced concrete per ml of spillway. As the area of reservoir is usually (in m²) between 5000 and 50000 times the spillway length in m (most often between 10 and 20000), the increase of live storage will be usually between 5 and 10000 m³ per m³ of reinforced concrete. Taking in account the usual costs in Africa and the extra cost for lowering the sill, the usual cost per extra m³ of storage will be in the range of 5 cents of U.S. $ (much less than the relevant cost for a new dam).

The expense will be made in local currency. For hydropower dams, this solution will be also usually very cost effective, the cost for reducing by half the nappe depth being in the range of 100 $ per m³/s of the existing spillway capacity.

For rather small reservoirs it is also possible to use flashboards (fig.4) of which the cost is very low. As the water level bending the elements is not know precisely, it is advisable that the height of the flashboard, i.e. the saving be less than 25% of the gap between the embankment crest and the existing sill and that all flashboards should bend for a reservoir level well under the embankment crest. The saving will thus be usually limited to O^m,50 or 1 ^m, well under saving by a P.K. Weir. But for small spillways and reservoirs the cost of this solution is very low.

5. INCREASING THE CAPACITY OF FREE FLOW SPILLWAYS:

It is usually possible to lower the sill and to build a P.K. Weir keeping the same storage level but multiplying by 2 or 3 the spilling capacity. This will require 0,5m³ of reinforced concrete for increasing by 1m³/s the spillway capacity at same reservoir level.

For this purpose, lowering the sill and placing concrete fuseplugs will be even less expensive : This solution require 0,5m³ of ordinary concrete per extra m³/s. a first fuseplug would tilt for the present design flood level and the last one for the present check flood.

If the dam is not very long, a parapet wall upon the embankment crest may be associated with the solutions hereabove.

6. INCREASING THE SPILLING CAPACITY OF GATED RESERVOIRS

Beyond a limited improvement by a parapet wall upon the embankment crest, various solutions of additional or emergency spillway may be used.

Instead of a costly gated emergency spillway which would not solve the general problem of gates jamming the most attractive solution could be a P.K. Weir used for exceptional floods or in case of gates jamming. Using the free board margin it may spill for instance 50m³/s/ml under a nappe depth of 4^m, requiring, 0,5m³ of reinforced concrete per extra m³/s.

It may be also possible to line the downstream slope of the embankment (fig.3) with R.C.C. where its height is of 5 to 10^m ; the necessary quantity of roller compacted concrete may be 1 or 2m³ per extra m³/s. This may be associated with fuse plugs upon the R.C.C.

CONCLUSION

For most new dams using P.K. Weirs will be much less expressive than traditional solutions and safer than fully gated spillways.

For many existing dams it is possible to use various low cost solutions (P.K. Weirs, parapet walls, slope lining, flashboards, fuseplugs) for increasing efficiency and safety.

These solutions may usually be implemented by local resources in local currency.

LEMPERIERE F. is honorary chairman of the French Committee on Large Dams and has been chairman of ICOLD Committee on Costs (1991-2001) He is chairman of Hydrocoop, a non profit making organization for technical informations on dams.

(E.Mail: hydrocoop_france@compuserve.com )

DE SAVIGNAC X. has been involved in very large hydraulic schemes in Africa : Cabora Bassa on Zambezi ; JONGLEI Canal on White Nile. He is General Secretary of Hydrocoop.